Mayer first learned she was pregnant in January. In mid-June, Yahoo's headhunters called Mayer to ask if she was interested in becoming CEO. She disclosed her condition to Yahoo's board. None expressed any issues.

That's a huge step forward for corporate America. In 1992, Lawrence Perlman, then the CEO of IT services firm Ceridian, gave a speech asking, "A Pregnant CEO: In Whose Lifetime?"

In ours, that's whose.

Mayer's planning to take a maternity leave of a few weeks and work throughout. That's in character for the famously workaholic executive.

Mayer's husband, Zack Bogue, is a venture capitalist who invests in big-data startups.

One note: Mayer is exceptionally good at keeping secrets.

We asked Mayer directly about her pregnancy last week by email. She didn't reply. We asked a number of sources who either expressed total ignorance or utter surprise. One typical reaction: "No way!"

And watch below to find out what lies ahead for Yahoo and Marissa Mayer: